The Deranged Umbreon

On our wikia we have Userboxes, I believe Wikipedia has these
too. I was wondering if anyone knew how to make these userboxes,
and if so, can you please teach me? I want to make one but they
always end up WAY to big. Here is an example of some userboxes:
http://minecraftpocketedition.wikia.com/wiki/User:The_Deranged_Umbreon
Under the section, Userboxes.

I am working on my Wikia and I wanted to change the background,
the background at the time was 1920X1080, so I went with a
background the same size since the old one fit so well. However,
when I found another picture (One 1920X1080) everytime I tried to
apply it in the ¨Change theme¨ section on the admin panel, it would
say ¨Incorrect file size¨ Can anyone please explain what I'm doing
wrong?

I am working on my Wikia and I wanted to change the background,
the background at the time was 1920X1080, so I went with a
background the same size since the old one fit so well. However,
when I found another picture (One 1920X1080) everytime I tried to
apply it in the ¨Change theme¨ section on the admin panel, it would
say ¨Incorrect file size¨ Can anyone please explain what I'm doing
wrong?

I'm trying to edit the main page of my Wikia and I wanted to add
things that update like ¨There are on this Wiki!¨ and so I'd just
put in the NUMBEROFARTICLES } template, but what i'm wondering is,
I have numberofarticles numberofactiveusers and numberofedits, are
there any more?